1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and an image display apparatus. In particular, it relates to methods of reducing the occurrence of false edges in a digital image by gray scale interpolation when the gray scale of a digital image is expanded by bit extension.
2. Description of the Related Art
In recent years, the brightness of the display screens of television sets and monitors has been increasing. As brightness increases, the amount of change in brightness per gray level in the gray scale increases. Ten-bit liquid crystal panels and other display devices with high gray-scale resolution are being developed, but digital television broadcasting uses an eight-bit gray scale, so despite the ten-bit resolution of the latest display devices, the effective resolution of the gray scale in a television picture remains eight bits. Not only does this prevent full utilization of the features of these new display devices; picture signals that should grade smoothly in color or brightness, such as pictures of a sunset or the surface of the sea, may instead grade in a staircase pattern, skipping one or more intermediate gray levels at each step and thereby producing visible false edges.
When only one level is skipped, the false edge can be eliminated by an averaging process. To deal with jumps between two gray levels that differ by N levels, where N is any integer greater than one, Japanese Patent Application Publication No. 10-84481 (Paras. 0002 to 0006 and 0025 to 0028, FIGS. 1 and 2) defines an interpolation region around the jump, separates the interpolation region into (N+1) discrete subregions, and assigns sequential gray levels to the subregions.
A general problem of such methods is that they fail to distinguish between true image edges and false edges caused by bit extension. If used to smooth out false edges, these methods also smooth out real edges, leading to a loss of sharpness, as if the picture were slightly out of focus.